Inverse Hall-Petch like behaviour in a mechanically milled nanocrystalline Al5Fe2 intermetallic phase

In the present investigation, the indentation study on the high-energy ball-milled nanocrystalline Al5Fe2 intermetallic compound has established the inverse Hall-Petch (IHP) behaviour. The structural characterisation of the milled powder particles by X-ray diffraction (XRD) and transmission electron microscopy has shown the evolution of nanocrystalline phase. Micro-indentation measurements have revealed the increase in hardness with decreasing grain size, reaching to a maximum of 9.0 +/- 0.3GPa up to a grain size of 32 +/- 4nm, followed by a decrease. The decrease in hardness with further refinement, an indication of grain size softening, demonstrates the IHP-like behaviour. The deviation from the Hall-Petch behaviour has been discussed using various models based on the dislocations and grain boundary-mediated processes. From the analysis, it appears that the model based on mesocopic grain boundary sliding phenomena is more appropriate to account for the observed grain size softening.